142 ' U. S. BUREAU OF FISHERIES * 
Air velocity —Throughout the free spaces over and under the flakes . 
of fish the velocity should be as high as can be used without the force 
of the air moving the fish. A velocity of at least 2,000 feet per 
minute can be used without difficulty, and it is recommended that 
the velocity be this or slightly higher. Even higher velocities would 
be desirable, and it may be possible to use them up to about 2,500 
feet per minute for cooking large oval-size fish. Small quarter-oils, 
however, tend to be blown about when the velocity is much greater 
than 2,000 feet per minute. 
In addition to increasing moisture loss there are two important 
advantages in using high-velocity air that have not yet been discussed: 
1. Large quantities of heat are needed to raise the temperature of 
the flakes and fish and to vaporize the water. Now, the higher the 
air velocity, the greater is the amount of air, and consequently heat, 
that can be put into a given space per unit of time. A tunnel does 
not have to be nearly as large, then, to handle a given quantity of fish 
at a velocity of, say, 2,000 feet per minute as at 1,000. Size and cost 
of equipment can be kept down in this way. Of course, the same 
quantity of heat can be put into the tunnel with low-velocity air 
by raising its temperature. This, however, proved unsatisfactory. 
2. Air, especially when mixed with considerable water vapor, 
makes a light gas when heated to about 300°. Being light, it is hard 
to force it through a long tunnel evenly because of its tendency to 
rise and collect at the top. The higher the velocity, however, the 
less trouble there is in this respect. 
Air temperatures.—In addition to high velocity, the air should 
have as high a temperature as possible. In this way the fish can be 
prepared most quickly. For large, pound-oval sardines a temperature 
around 300° should be used for preparing the fish, with an air velocity 
of 2,000 feet per minute. These conditions remove about the right 
amount of water from the average run of fish in about 15 minutes. 
For quarter-oil fish the same velocity and time are about right, but a 
temperature around 275° is better. Three-quarters mustard fish 
are prepared satisfactorily at either temperature. Exact recom- 
mendations can not be given in either case, as it is necessary to vary 
the temperature and time to meet different conditions. The fish 
may be extra large, rather small, or mixed, and they may be lean or 
fat. Different canners have different ideas as to how dry they wish 
their fish to be. All of these conditions can be cared for easily. A 
little experience, which can be gained quickly, is all that is necessary 
to enable an operator satisfactorily to meet the various conditions 
as well as the time factor. 
In large-scale operations it will not be practicable to maintain the 
same temperature throughout the tunnel or chambers in which the 
fish are cooked. The air will enter at one end, say, of a tunnel full 
of fish, and go out the other. The temperature will drop as it 
passes through, due to the heat that has been taken from the air by 
the fish, the flakes, and the rest of the equipment. Under operating 
conditions this drop in temperature will be quite uniform. A drop 
of 50° to 75° is not too much for good results if an average tempera- 
ture of 275° to 300° is maintained. For example, in the case of a 50° 
drop the air will enter the tunnel, say, at 325° and leave at 275°, 
giving an average temperature of 300°. The flakes of fish will pass 

